This invention relates generally to optical elements and more particularly to coatings which protect and strengthen IR optical elements.
As is known in the art, optical imaging systems generally include an optical element which shields the remainder of the imaging system from a hostile environment. For example, with infrared (IR) airborne imaging systems, an IR transparent optical element, such as window or dome, is mounted on the airborne system to isolate the remainder of the IR imaging system from exposure to humid, corrosive, abrasive, and high temperature environments. Prolonged exposure to these environments generally degrade the optical and physical characteristics of the material of the optical element. For many applications involving low speed missiles, low velocity water droplet impact is generally the most severe environmental exposure. However for newer hypersonic missile applications, the thermal heating of the optical element is the most severe environmental exposure. With missiles travelling at hypersonic velocities (e.g. MACH 1+ to MACH 4 or higher) the optical element heats up very quickly, and optical elements comprised of materials with low thermal shock resistance are particularly susceptible to thermally induced damage.
A second application of such elements is as an observation window in a furnace to monitor a combustion process. Again, these elements shield an IR system from a high temperature environment.
Typically, materials which offer the best mechanical durability and optical performance for infrared imaging systems, particularly in the 8 .mu.m to 12 .mu.m infrared band, are limited to a relatively small number. Suitable materials include zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride and cadmium telluride. Ternary sulfide materials such as calcium lanthanum sulfide are also currently being developed for IR applications, particularly in the 8-12 .mu.m band. These ternary sulfide materials may provide some improvement in durability but even these materials are susceptible to the environmental exposures mentioned above. Generally, all of the aforementioned materials are relatively brittle, have low thermal shock resistance, and have a relatively low resistance to damage, particularly damage sustained during high velocity water droplet impact.
Moreover, these materials also have relative high susceptibility to forming oxides while exposed to high temperature environments containing oxygen as would be encountered in the flight of a hypersonic missile, or as an observation window in a furnace. These oxides generally reduce significantly the IR transmittance of the optical element, increase IR scatter and in general degrade optical characteristics. In particular, many of the materials form oxides which have significant absorption bands in the 8-12 micron range.
It is also known in the art that optical energy incident upon a surface of an optical element will result in reflection of energy at such surface if the index of refraction of the material comprising the optical element is significantly different than the index of refraction of the medium from which the energy originates. Generally, for airborne systems, the originating medium is air having an index of refraction of about 1.0. Accordingly, it is standard practice in the optical industry to provide coatings of material of appropriate refractive index over the incident surface of the optical element to reduce such reflection losses. At the deposited thicknesses, which are generally related to a fraction of an optical wavelength, these coatings are transparent in the IR band. However, heretofore such optical coatings have served only to reduce reflection losses caused by a mismatch in refractive indices. Such optical coatings have not increased the thermal shock resistance of the optical element nor have such coatings reduced or eliminated the aforementioned problems of oxidation of the material of such optical elements.
Thus, for the above-mentioned applications, a suitable coating to protect such elements should increase the thermal shock resistance of the material, while reducing thermally induced oxidation of the material. Such a coating should also provide proper anti-reflection correction and maintain high IR transmittance.
Moreover, in applications where rapid thermal cycling is encountered, the coating would need to have coefficient of thermal expansion characteristics which are substantially the same as that of the base material. This would retard debonding of the coating from the base during exposure to high temperature environments.